1. Field of the Invention
The invention relates to the field of microelectronic calculator/controllers and more particularly to a structure for an easily testable single chip calculator/controller.
2. Prior Art
Single chip calculator/controllers and microcomputers are well-known in the art. Such devices can range from relatively simple calculator chips with fixed storage and microroutines up to very powerful and flexible microcomputers capable of handling data processing functions or for use as a resident controller in a large computer system.
The logic organization or architecture chosen for a particular device dictates the size and complexity of fabrication of the resultant chip and the flexibility of its microprogrammed instructions. The more powerful and flexible the logic functions desired, the greater the complexity of the resulting chip. However, with increasing complexity comes increased cost in fabrication and in testing. Oftentimes an entire chip must be discarded because of a single flaw in its fabrication.
Prior art calculator/controller systems generally have included a program storage unit (usually a read only memory) which contains an assembly of instructions, active storage elements (shift registers or a random access memory) the contents of which can be modified by the instructions contained in the program storage unit, and a processing unit which sequences the instructions and modifies the data contents of the active storage elements as specified by the instructions. In prior art devices, the active storage elements are dedicated to specific functions and require specific hardware, control signals, and associated interconnections for their operation.
Testing of LSI calculator chips poses formidable problems: the complexity of the circuitry and interconnections makes diagnostic testing a time consuming affair since literally thousands or millions of possible operating states exist in today's digh density LSI chips. Ease and quality of testing is directly related to the capability to read out and write the status of the various internal storage elements of an LSI calculator/controller chip. One such common prior art method uses a test pin to route the state of the internal storage elements to output pins by means of a multiplexer controlled by the test pin, and route the state of input pins to the active storage element by the same means. This technique, however, cannot be used when the number of active elements is high, as is the case in a single chip calculator/controller.